1. Field of the Invention
The present invention relates to an active matrix substrate that is used in such display devices as liquid crystal display devices.
2. Description of the Related Art
FIG. 14 shows a conventional arrangement of an active matrix substrate used in liquid crystal display devices. As shown in FIG. 14, an active matrix substrate 100 includes a plurality of scanning signal lines 116 and a plurality of data signal lines 115, such that the scanning signal lines 116 and the data signal lines 115 are disposed to cross each other, TFTs (Thin Film Transistors) 112 that are formed in the vicinity of the intersection points of both signal lines (115 and 116), and pixel electrodes 117. A source electrode 119 of the TFT 112 is connected with the data signal line 115, and a drain electrode 108 of the TFT 112 is connected with the pixel electrode 117 via a drain lead electrode 107. The scanning signal line 116 also functions as a gate electrode of the TFT 112. A hole is created in an insulating film disposed between the drain lead electrode 107 and the pixel electrode 117, which hole forms a contact hole 110 that connects the drain lead electrode 107 and the pixel electrode 117. The pixel electrode 117 is a transparent electrode (made of ITO or the like) arranged such that light (backlight) from beneath the active matrix substrate can pass through.
The active matrix substrate 100 is arranged such that a scanning signal sent through the scanning signal line 116 (gate ON voltage) turns on the TFT 112 (that is, TFT 112 is put in a state of allowing a current flow from the source electrode 119 to the drain electrode 108). When the TFT 112 is ON, a data signal sent through the data signal line 115 (signal voltage) is written on the pixel electrode 117 via the source electrode 119, the drain electrode 108 and the drain lead electrode 107. A storage capacitor (Cs) wiring 118 has a function that it prevents self-discharge of a liquid crystal layer while the TFT 112 is off, and the like function.
That portion of the pixel electrode 117 beneath which the drain lead electrode 107 or the storage capacitor wiring 118 is formed does not contribute as an optically transparent portion because the drain lead electrode 107 and the storage capacitor wiring 118 (metal) block light. Therefore, taking into consideration improvement in the open area ratio alone, it is preferable that the portion of the drain lead electrode 107 beneath the pixel electrode 117 should be as small as possible. However, with a small drain lead electrode 107, misalignment between the drain lead electrode 107 and the contact hole 110 is more likely to occur. This misalignment leads to increased contact resistance and, consequently, causes poor display quality such as decreased response speed and the like.
Japanese Unexamined Patent Application Publication No. 2004-144965 discloses an arrangement, according to which the open area ratio is increased in displaying images with backlighting in a semi-transparent liquid crystal display device. A contact hole region in a semi-transparent liquid crystal display device, despite functioning as a reflective element, displays differently from other reflective elements (partly because an interlayer film does not exist in this part). By creating a non-electrode region (a transparent portion) on a drain electrode in a contact hole, the open area ratio is increased in displaying images by a transmissive display method.
However, with a non-electrode region (a transparent portion) created on a drain electrode in a contact hole as described above, as the contact area between the drain electrode and a pixel electrode decreases, a change (decrease) in the contact area caused by misalignment is more likely to occur. This may be avoided by enlarging a drain electrode and a contact hole region as well, as far as semi-transparent liquid crystal display devices alone are concerned (because the portion in which the drain electrode exists is a reflective element, a larger drain electrode will not affect the open area ratio in displaying images in the semi-transparent liquid crystal display devices). However, this is not the case in transparent liquid crystal display devices. As explained above, an increased drain electrode area in a transparent liquid crystal display device directly leads to a decreased open area ratio.